1. Field of the Invention
This invention relates to apparatus and methods for optically processing x-rays. In particular, this invention relates to the use of spherical mirrors in grazing incidence to focus, image, collimate, and perform interferometry in the x-ray band of the spectrum. The present invention is particularly useful for the full range of x-ray imaging, especially for improving the quality of focus of the final image, and for x-ray lithography.
2. Description of the Prior Art
The value of the refractive index of materials in the soft x-ray band is slightly below one, and coupled to a high absorption coefficient. The high absorption has made all attempts at refractive x-ray optics unsatisfactory to date. Three approaches are used: zone plates, normal incidence multilayer mirrors, and grazing incidence mirrors.
The zone plate images through use of diffraction. Concentric rings are ruled on a thin sheet and diffract some of the radiation to the center where an image forms. The systems are typically inefficient due to the physics of diffraction, and the resultant image usually has severe chromatic aberrations.
Multilayer mirrors are made by depositing alternating thin layers of two elements with different indices of refraction. This creates constructive interference, and hence high reflectivity at one wavelength. The approach has the advantage that it can be used with normal incidence optics, but has the drawback of very limited spectral bandpass. Multilayers are used at wavelengths longer than about 4 nm because below this it is difficult to achieve adequate layer to layer coherence.
Grazing incidence optics make use of the fact that the index of refraction is below one, allowing radiation incident at a low graze angle to experience total external reflection. Grazing incidence mirrors also have the advantage that polish requirements drops as a function of sin.theta., where .theta. is the graze angle, avoiding the need for sub-nanometer surface quality, even well into the x-ray spectrum.
The first optical designs based on grazing incidence were described by Kirkpatrick and Baez (K-B) in 1948 (1951 patent). They used flats, spheres and cylinders to create a one dimensional line focus. The second dimension of focus is achieved by a second optic placed beyond the first, oriented at 90-degrees. This arrangement has severe comatic aberration that limits the utility in high resolution applications. It was not appreciated until now that two spherical mirrors for each dimension of focus could be selected and oriented to minimize coma and also spherical aberration.
In 1952, Wolter described a system of extreme aspherical paraboloids, hyperboloids, and ellipsoids that produced high resolution images on-axis and better off-axis resolution. Unfortunately, the difficulty and expense of manufacturing and aligning extreme aspheres has limited both the availability and ultimate quality of the optics.
One recent variation of this approach is to replace the paraboloid and hyperboloid of a typical Wolter with two toroids. This allows a diverging synchrotron beam to be collimated into a straight, narrow line with two grazing incidence reflections. A device of this nature is disclosed in U.S. Pat. No. 5,031,199 by Cole et al. However, the aberration control of toroids is significantly poorer than that of spheres, their fabrication cost is much higher, and their resultant optical fabrication quality is much lower in terms of figure and scatter.
A need remains in the art for apparatus and methods for optically processing x-rays inexpensively and without significant comatic or spherical aberrations.